Vacuum Leaks/Unmetered Air

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2010 Mercury Milan, 2010 Mercury Mariner, 2010 Mercury Grand Marquis, 2010 Lincoln Town Car, and 2010 Lincoln Navigator L. However, it is still accessible from the selected car via links, so may be relevant.

With this condition, the engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition can be caused by unmetered air entering the engine, or due to a MAF concern. In this situation, the volume of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. Vacuum leaks normally are most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates the fault occurred at idle, a check for vacuum leaks/unmetered air might be the best starting point.

For example, loose, leaking or disconnected vacuum lines, intake manifold gaskets or O-rings, throttle body gaskets, brake booster, air inlet tube, stuck/frozen/aftermarket positive crankcase ventilation (PCV) valve, and unseated engine oil dipstick.